Photodiode

ABSTRACT

The invention provides a method of manufacturing an avalanche diode comprising the steps of applying a mask ( 6 ) over an active diode region ( 5 ) in a wafer ( 1 ), and damaging the region the surrounding the active diode region by breaking bonds in the semiconductor lattice to provide gettering sites in this surrounding region.

This is a nationalization of PCT/IE2004/000072 filed 14 May 2004 andpublished in English.

INTRODUCTION

The invention relates to avalanche photodiodes such as those whichoperate in linear avalanche mode or single photon counting Geiger mode.

In existing avalanche diodes light interacts with a silicon lattice togenerate electron hole pairs which cause breakdown at a junction wherethere is a peak electric field. In the Geiger mode, there is a highapplied bias that accelerates the electrons and holes causing impactionisation and avalanche breakdown. It is desired that this effect onlyoccur by electrons and holes initiated by photons. However, thermallygenerated electrons and holes exist which cause spurious counts, knownas the dark count, in the absence of light. The presence of defectsincreases the dark count which is detrimental to device operation.

W. J. Kindt, “Geiger Mode Avalanche Photodiode Arrays for SpatiallyResolved Single Photon Counting”, Delft University Press, ISBN:90-407-1845-8, 1999 describes a photodiode in which polysilicon isdeposited over the silicon wafer within a top oxide ring. Thepolysilicon is doped with n-type dopant to provide an n-type cathodeover a p-type anode. The junction between the cathode and the anode isthe active area.

It is known to provide a guard band around the diode active region tospread out the electric field and prevent point concentrations at theedges. The guard band can be specifically implanted as a separate layeror combined into the cathode layer.

The invention addresses this problem.

STATEMENTS OF INVENTION

According to the invention there is provided a method of manufacturingan avalanche diode comprising the steps of;

-   -   applying a mask over an active diode region in a wafer,    -   damaging the region surrounding the active diode region by        breaking bonds in the semiconductor lattice to provide gettering        sites in this surrounding region.

In one embodiment the surrounding region is damaged by subjecting it todoping.

In another embodiment the surrounding region is damaged by subjectingboth the mask and the surrounding region to doping.

In one embodiment the method comprises the further step of heating tocause diffusion of cathode ions from the mask into the underlyingmaterial to form a cathode.

Preferably the mask is of a material which acts to draw defects from theunderlying material during the process.

In one embodiment the doping is performed so that a top layer of themask has a high concentration of defects which have diffused from theunderlying material.

In another embodiment the mask is of a polycrystalline material.

Preferably the mask is of polysilicon material.

In one embodiment the method comprises the surrounding region has thesame doping as the anode.

In another embodiment the method comprises the doping is performed byarsenic implanting.

In a further embodiment the method comprises the further steps ofgrowing an oxide ring around the diode active region.

In one embodiment doping is performed within the oxide ring.

In another embodiment the method comprises the further steps of applyinga bridge from the mask to the oxide to form an Ohmic contact for thediode.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be more clearly understood from the followingdescription of some embodiments thereof, given by way of example onlywith reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic cross-sectional elevational view showingproduction of a Geiger mode avalanche photodiode;

FIG. 2 is a similar view when fabrication is complete; and

FIG. 3 is a plan view of the detector.

Referring to FIG. 1 a p-type silicon wafer 1 is provided with p+ contactregions 2 at its outer upper surface. An oxide ring 3 is grown aroundthe upper surface of the wafer 1, leaving a circular central area forproducing the diode active regions.

At the centre of this area an enhanced p-type initial anode region 5 isprovided, the doping of this region being approximately two orders ofmagnitude greater than that of the underlying wafer 1. A disc-shapedpolysilicon layer is then applied over the p-type anode 5 by depositionand etching. The thickness is approximately 200 nm thick, and thediameter is such as to be greater than that of the underlying p-typeanode 5.

The area within the oxide ring 3 is then subjected to arsenicimplanting. There is then heat treatment.

The arsenic implanting and the subsequent heat treatment cause thefollowing activities to occur during manufacture:

-   -   (a) the polysilicon 6 acts as a barrier to prevent n-type doping        of the bulk of the initial anode region 5 by the arsenic        implant, except in a shallow top surface layer,    -   (b) there is n+ diffusion into this shallow layer, this        diffusion being from the polysilicon 6 during heating and from a        surrounding region 10,    -   (c) the surrounding region 10 is doped directly by the arsenic        imlant 7, thus providing a guard band to avoid point        concentration at the sides in the end-product device,    -   (d) the surrounding region 10 is highly damaged by the arsenic        implanting due to the presence of the polysilicon 6, and thus it        provides a substantial number of gettering sites for substantial        migration of defects from the region below the polysilicon,    -   (e) the polysilicon itself, due to its grainy crystalline        structure, also provides gettering sites for migration of        defects from the underlying material.

Referring again to FIG. 2, the result of these activities is that thepolysilicon 6 transforms to a hybrid structure 11 comprising a lowerregion 12 of n+ doping arising from arsenic implanting of thepolysilicon 6 and migration of n+ into the top of the initial anoderegion 5, and an upper region 13 rich in defects. The region 13 arisesbecause of the migration of defects from underneath.

As shown in FIG. 3 the avalanche diode is completed by applying apolysilicon bridge 19 to a metal Ohmic contact 20 on the oxide 3. Thisarrangement avoids the possibility of the Ohmic metal such as aluminiumgiving rise to diffusion of ions into the underlying p-type materialwhich might occur if the contact were directly on the p-type material.

It will thus be appreciated that the polysilicon 6 and the processingsteps give rise to several advantages simultaneously during production.The region 10 around the active diode regions is particularly damagedbecause of presence of the polysilicon 6 acting as a mask for thecentral disc region. The damaged region 10 actively draws defects awayfrom the active regions, and at the same time the polysilicon itselfalso draws defects upwardly. This double attraction of defects helps toachieve a particularly pure, defect-free diode active region. Also, thepolysilicon effectively merges with a shallow top layer of the initialanode region 5 to form the cathode 12 by diffusion of n+ downwardlyduring heating. Furthermore, the polysilicon can then provide an Ohmiccontact.

The invention advantageously enables the use of a polysilicon layer toform the cathode region, cathode contact, guard band, and provide a areaof increased defect removal at the edges of the guard band.

In the embodiment described doping is used to damage the regionsurrounding the active diode region this is because this doping achievesmultiple purposes of doping the region under the mask to form thecathode and at the same time damaging the surrounding region, it istherefore very efficient. However, it is envisaged that the surroundingregion may be damaged in any other appropriate manner which selectivlybreaks bonds in the semiconductor lattice in this surrounding region forexample, this may be achieved mechanically.

The invention is not limited to the embodiments described but may bevaried in construction and detail.

1-13. (canceled)
 14. A method of manufacturing an avalanche diodecomprising the steps of; applying a mask over an active diode region ina wafer, damaging the region surrounding the active diode region bybreaking bonds in the semiconductor lattice to provide gettering sitesin this surrounding region.
 15. The method as claimed in claim 14,wherein the surrounding region is damaged by subjecting it to doping.16. The method as claimed in claim 14, wherein the surrounding region isdamaged by subjecting both the mask and the surrounding region todoping.
 17. The method as claimed in claim 14, comprising the furtherstep of heating to cause diffusion of cathode ions from the mask intothe underlying material to form a cathode.
 18. The method as claimed inclaim 14, comprising the further step of heating to cause diffusion ofcathode ions from the mask into the underlying material to form acathode; and wherein the mask is of a material which acts to drawdefects from the underlying material during the process.
 19. The methodas claimed in claim 14, comprising the further step of heating to causediffusion of cathode ions from the mask into the underlying material toform a cathode; and wherein the mask is of a material which acts to drawdefects from the underlying material during the process; and wherein thedoping is performed so that a top layer of the mask has a highconcentration of defects which have diffused from the underlyingmaterial.
 20. The method as claimed in claim 18, wherein the mask is ofa polycrystalline material.
 21. The method as claimed in claim 18,wherein the mask is of polysilicon material.
 22. The method as claimedin claim 14, wherein the surrounding region has the same doping as theanode.
 23. The method as claimed in claim 14, wherein the surroundingregion is damaged by subjecting it to doping; and wherein the doping isperformed by arsenic implanting.
 24. The method as claimed in claim 14,comprising the further steps of growing an oxide ring around the diodeactive region.
 25. The method as claimed in claim 14, comprising thefurther steps of growing an oxide ring around the diode active region;and wherein doping is performed within the oxide ring.
 26. The method asclaimed in claim 14, comprising the further steps of growing an oxidering around the diode active region; and comprising the further steps ofapplying a bridge from the mask to the oxide to form an Ohmic contactfor the diode.